1. Field of the Invention
The present invention relates to an algorithm and a direct current (DC) microgrid charge/discharge system with the algorithm that noticeably increases charge/discharge voltage during charging and discharging by connecting a plurality of secondary batteries in series, compared to a single battery, that increases power conversion efficiency of a DC-to-DC (DC/DC) converter for charge/discharge power control by setting an appropriate voltage of a DC microgrid that directly uses discharged electricity in a DC state to perform charging to be about twice a maximum voltage of a battery group, that increases power conversion efficiency of the DC microgrid charge/discharge system by configuring a DC energy storage system (ESS) with a combination of a typical electrolytic condenser and a super condenser or a group of the secondary batteries, and that simplifies the DC microgrid charge/discharge system.
2. Description of the Related Art
A microgrid refers to a localized power supply system.
Large-capacity lithium secondary batteries used in electric vehicles each have a relatively low operating voltage, for example, a voltage of about 2 volts (V) to about 4.7 V, and a relatively high operating current, for example, a current of about 25 amperes (A) to about 50 A.
In an electric vehicle, tens to hundreds of large-capacity lithium secondary batteries are connected and used. In a process of manufacturing a battery assembly, a battery of a unit module (for example, a single battery, or a few batteries are connected in parallel or in series) needs to be charged and discharged at a maximum capacity. However, there are several technical difficulties, due to low conversion efficiency for converting electric energy of a battery with a relatively low voltage to an alternating current (AC) of a nationwide grid with a relatively high voltage, and for recycling the electric energy.
In a related art, in a system for simultaneously charging and discharging a large-capacity secondary battery in a switch mode, a single lithium secondary battery with an operating voltage of about 2 V to about 4.7 V may be discharged. Electricity obtained by the discharging needs to be regenerated in a nationwide AC grid by consuming the electricity as heat, or by converting the electricity to AC electricity.
A direct current (DC)-to-AC (DC/AC) converter used to regenerate power may have extremely low conversion efficiency, for example typically 40%, since a voltage of a single battery is considerably lower than a voltage of a commercial AC grid (for example, 220 V or 380 V). During conversion, remaining energy may be consumed as heat in the DC/AC converter.
Accordingly, a process of using a conventional charge/discharge apparatus is known to use the most power in a factory of manufacturing batteries including conditioning equipment for cooling.
In a typical apparatus for charging/discharging a battery, a constant current (CC) scheme, a constant voltage (CV) scheme, or a constant power scheme may be performed. In a large-capacity battery, the CC scheme may be mainly used.
A secondary battery charging and discharging system for enabling AC regeneration and DC regeneration in charging and discharging of a secondary battery, and an operation method thereof are disclosed in Korean Patent Publication No. 10-2012-0103337 as a related art. On the other hand, in the present invention, a plurality of secondary batteries may be connected in series, and power conversion efficiency may be increased using a bidirectional DC/DC converter and a constant current source. Additionally, in the present invention, a charge/discharge system may be miniaturized, and charging and discharging may be performed regardless of the number of batteries connected in series. Thus, Korean Patent Publication No. 10-2012-0103337 is significantly different from the present invention.
A technical configuration for enhancing accuracy of charging control of a secondary battery with a less change in a terminal voltage based on a change in a state of charge (SOC) is disclosed in Korean Patent Publication No. 10-2010-0119574 as a related art. On the other hand, in the present invention, a plurality of secondary batteries may be connected in series, and power conversion efficiency may be increased using a DC/DC converter and a constant current source. Additionally, in the present invention, a charge/discharge system may be miniaturized, and charging and discharging may be performed regardless of the number of batteries connected in series. Thus, Korean Patent Publication No. 10-2010-0119574 is significantly different from the present invention.
Additionally, a technical configuration including discharge route circuits for each of secondary batteries connected in series and for protecting overcharge is disclosed in Korean Patent Publication No. 10-2010-0122911 as a related art. However, because each of the secondary batteries includes a separate discharge resistor and a switching element to form a discharge route circuit, power consumption, and manufacturing costs may increase.